Rudder on long keel

Discussion in 'Boat Design' started by pitbull, Jan 26, 2022.

  1. pitbull
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    pitbull Junior Member

    I've been reading a few old books regarding long keel boats (like channel pilot cutters) and some advocate a vertical rudder post (when viewed from the side) and others a rudder post that rakes forward the deeper it goes.

    For example, advocating a strongly raked sternpost (45 degrees) the designer of Jolie Brise, Paumelle, is supposed to have said: [when] There's a sea coming up behind you and your bow's dipped and the stern comes up, that's the time you want your rudder to be most effective.

    Hiscock seems to suggest that the only benefit of a raked rudder is to remove some of the deadwood and reduce wetted surface. Indeed if the rudder is raked too much it gives difficulty steering (30 degrees should be fine).

    I don't have enough sailing experience to know, how much rake is desirable and why ? Raked to trim underwater area and to move the rudder farther under the boat to mitigate the stern rising to a wave ? Or vertical to keep the turning force farther aft ? What are your thoughts please ?
    transom-raked.png straight-bustle.png
     
  2. wet feet
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    wet feet Senior Member

    Nothing to do with rudders,but I would suggest that no boatyard would be particularly pleased to see a boat that posed such a challenge to chock up ashore.A straight portion of the keel would make their lives a bit more straightforward and in consequence,your bills would be a bit smaller.

    A couple of other things stand out from the images:the mast is a very long way aft and what the old time designers referred to as the master section has a lot of rake in plan view.

    The normal design spiral begins with the requirements of the boat and it's function and then uses the parameters thus defined to give a starting point for an initial version,which then goes through a number of iterations.What particular duty is the boat intended for?
     
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  3. gonzo
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    gonzo Senior Member

    The application should drive the design. For example, a boat that is hauling nets may benefit from a rudder that is not protruding from the stern.
     
  4. mc_rash
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    mc_rash Senior Member

    @pitbull could you provide us with a section view?
     
  5. mc_rash
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    mc_rash Senior Member

    In my eyes the innerst buttock line could be moved little upwards and thus also the keel which would lead to reduced wetted area/resistance. Also compared to old pilot cutters the keel still looks big IMHO.
     
  6. philSweet
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    philSweet Senior Member

    I think you are probably looking at the wrong end of the machine. If you need a 9 foot long tiller to steer the boat, it's a lot easier to find room for it if the rudder is raked.
     
  7. gonzo
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    gonzo Senior Member

    What difference does the rake make? It would be the same distance from the stern and take the same amount of deck space.
     
  8. philSweet
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    philSweet Senior Member

    If the keel is the same length, rake puts the rudder head further aft. That way, a big tiller doesn't intrude as far.
     
  9. gonzo
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    gonzo Senior Member

    If the boat is longer, a tiller of equal length leaves more deck space. However, that is not a function of the rake, but the deck length.
     
  10. pitbull
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    pitbull Junior Member

    @wet feet Thanks for the practical suggestion about chocking up ashore. yes the master section rake was my intention - perhaps I overdid it. Per Skene, the traditional boat keel also seem to like to rake down from bow to stern continuously so that the leading edge is constantly entering solid water all along its length.

    I have no idea where to put the mast yet - I just found I needed to add one or I couldn't picture the shaded isometric projection of the boat on the bottom right of the lines plan properly.

    @mc_rash Here are the lines - I have not spent time on fairing yet (I don't know what the stern will look like even) and I have not incorporated any of your suggestions yet. I'm also just learning how to get delftship to do my bidding.
    10m long, 3m beam, 1.65m draft, 10 tonnes

    I found some comments on rudders by Skene - he writes:

    The raking rudder post gives the best shape of lateral plane and neutralizes the lifting component which a rudder with vertical post has when yacht is heeled.

    I see his point about removing some deadwood, but I'm not sure I can visualize why it makes a difference to the lifting component, maybe it induces less yaw with the same force closer to midships. He seems to like bow hung rudders too !

    Raking the rudder post and pivoting the whole thing aft of the transom seems to be the way to go (Although I have grown fond of her short stern overhang - perhaps not very pilot cutter). The prominent forefoot seems to be a cutter feature, better for heaving-to. I don't doubt it, but never having hove-to I don't have a visceral feel for it. Something to do with the boat being harder to yaw perhaps.

    Maybe it's time for me to book a sailing holiday on one of those old working boats and get a feel for things.

    lines1.png
     
    Last edited: Jan 28, 2022
  11. pitbull
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    pitbull Junior Member

    Oh- and you asked what the boat was for. I have long held on to the dream of building a boat and sailing her across an ocean or two. I had imagined there would be two capable crew on the boat (myself and one other). I am not averse to buying plans but it would be pretty cool to be involved in the whole design. I want to do all this within the next 5 years before I get too old.

    After I have got that out of my system I'd like something in which I can take my kids (and the grandkids if I am luck enough to get any) out for the occasional sail up and down the coast. It would essentially double as a "cabin on the coast" if I can find a suitable mooring. I live about an hour's drive from the Oregon coast.

    I'd like a boat that can be easily destroyed at the end of its life, or, if I mess up the crossing, won't leave mounds of plastic waste in the ocean.

    I am not particularly concerned about resale value, although if it turns out to be worth something I'll accept the offer ! I had not seen this as an investment or a luxury yacht.

    I want something low maintenance - within the limits of what is feasible in that regard. So few frills.

    I like the traditional look and there's a lot of living space in it for its length. This is not a reconstruction, I'd like to make sensible use of modern cordage, professionally made sails etc. It does not need to have hand-sewn tanbark flax sails and hemp sheets.

    I understand the arguments about "sailing fast enough to escape danger" but I prefer something that could stand up to a blow if I were offshore in the wrong place at the wrong time and so I have convinced myself that, other than a desire that it is able to go upwind tolerably well, I have no particular need for a fast boat. The whole point is to make the trip, if I just wanted to get to the destination I would fly there.

    I have had fun sailing dinghies and catamarans, if I feel the need for a white knuckle ride I'll go back to one. I will need to pick up a lot more sailing skills to be able to go offshore, that will be part of the fun

    I need a boat I can feel confident in, a gentle-ish motion, wide side decks, good stanchions and handholds, strong and waterproof topsides, ideally a less-homicidal boom.
     
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  12. Rumars
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    Rumars Senior Member

    I wouldn't say your model has a deep forefoot, rather a cut away one. A full keel with a deep forefoot is a boat with a straight stem, like for example a fifie. Anyway, rudder rake is a result of a keel hung rudder and a decision of how much of a skeg you wish to retain. Less skeg results in better maneuverability, more skeg in better directional stability. The goal is to find a balance between this two.

    Unless you really wish to learn how to design a boat from scratch, I see no reason for you to do it. There are many designs for this kind of boat in the size you want. For example Atkin&Co. has Tally Ho Major, Nutmeg, Pam and Jonquil. Plan prices range from 200 to 400$ (altough they appear to not ship momentarily), cutter, yawl ketch, with or without counterstern.
    This are old plans, for carvel, without support, you will very much be a part of "designing" it, you need to loft, fair, decide on building techniques, structural details, the entire interior plus the deck plan. You just skip the part where you have to learn how to do all the boring math about weight distribution, sail and lateral area balance, structure, etc.
     
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  13. wet feet
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    wet feet Senior Member


    The sailing holiday might be fun.I think Norman Skene must have written his comments about lifting components when his mind was otherwise occupied.If the boat is heeled and a rudder deflection is present,the rudder is likely to produce a lifting component unless lee helm is present.Regardless of the angle of the rudder post to keel,the angle between rudder post and centreline is always going to be zero and all that happens with a raked rudder post is that the moment applied is reduced because of the smaller distance to the CLR.Which might necessitate a bigger rudder and a correspondingly larger force generated.The further aft the rudder is located,the smaller it can be while still generating sufficient turning moment.There is much to be said in favour of a transom hung rudder,unless and until you back it into a harbour wall!It can be easier to unship for repairs and it is always easier to see if debris is present.Then there is the location of the tiller as far aft as possible
     
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